# Sum won't seem to add the fields?!

**URL:** <https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461>\
**Category:** Rails\
**Created:** [August 26, 2009, 11:46pm UTC](https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461 "2009-08-26T23:46:26Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/dbc845/32.png) [@Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/u/Ruby_on_RailsS_Talk)\
**Post date:** [August 26, 2009, 11:46pm UTC](https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461/1 "2009-08-26T23:46:26Z")

</div>

I have the code which creates the following made up of an array of 3  
TreatLists each with a number of Treatlistitems (I’ve broken it up to  
make it easier to read).

?\> @breakdown  
=\> [  
#\<Treatlist:0x23b98ac @items=[  
#\<Treatlistitem:0x2389ef4 @longname=“Eye Brow Shape”, @spend=17.0,  
@prodtreat=“T”, @numsold=2, @unitcost=10.0\>]\>,

#\<Treatlist:0x235df34  
@items=[#\<Treatlistitem:0x235b16c @longname=“Back Massage”,  
@spend=10.0, @prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
#\<Treatlistitem:0x23597cc @longname=“Hopi Ear Candles”, @spend=10.0,  
@prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
#\<Treatlistitem:0x23582dc @longname=“Back Massage (Essential Oils)”,  
@spend=10.0, @prodtreat=“T”, @numsold=1, @unitcost=10.0\>]\>,

#\<Treatlist:0x2357a1c @items=[  
#\<Treatlistitem:0x2351b30 @longname=“Back Massage”, @spend=10.0,  
@prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
#\<Treatlistitem:0x234f9fc @longname=“Hopi Ear Candles”, @spend=10.0,  
@prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
#\<Treatlistitem:0x234cf90 @longname=“Eye Brow Shape”, @spend=10.0,  
@prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
#\<Treatlistitem:0x2349db8 @longname=“Full Leg Wax DSS”, @spend=18.0,  
@prodtreat=“T”, @numsold=2, @unitcost=10.0\>,  
#\<Treatlistitem:0x23476bc @longname=“Bridal Make Up”, @spend=10.0,  
@prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
#\<Treatlistitem:0x2342fcc @longname=“Collin Face Powder”, @spend=10.0,  
@prodtreat=“P”, @numsold=1, @unitcost=10.0\>]\>]

I’m trying to add together the ‘spend’ figures from each but only if  
prodtreat = T or P. When I try the following:

> > @breakdown[1].items.sum{|item| item.spend if item.prodtreat == “T”}  
> > =\> 30.0

it works!!!

But when I try it with P …

> > @breakdown[1].items.sum{|item| item.spend if item.prodtreat == “P”}

I get the following:

NoMethodError: You have a nil object when you didn’t expect it!  
You might have expected an instance of Array.  
The error occurred while evaluating nil.+  
from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/  
active\_support/core\_ext/enumerable.rb:63:in `sum' from (irb):65:in`inject’  
from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/  
active\_support/core\_ext/enumerable.rb:63:in `each' from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/ active_support/core_ext/enumerable.rb:63:in`inject’  
from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/  
active\_support/core\_ext/enumerable.rb:63:in `sum' from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/ active_support/core_ext/enumerable.rb:61:in`sum’  
from (irb):65

> >

I really have no idea why this is not working or not returning a zero  
value?! Any ideas please?

Cheers

Darren

---

<div class="post-metadata">

**Author:** ![Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/dbc845/32.png) [@Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/u/Ruby_on_RailsS_Talk)\
**Post date:** [August 26, 2009, 11:48pm UTC](https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461/2 "2009-08-26T23:48:12Z")

</div>

I have also noted that with @breakdown[2] it doesn’t work even though  
I know I have a “P” in there!

> > @breakdown[2]  
> > =\> #\<Treatlist:0x2357a1c @items=[  
> > #\<Treatlistitem:0x2351b30 @longname=“Back Massage”, @spend=10.0,  
> > @prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
> > #\<Treatlistitem:0x234f9fc @longname=“Hopi Ear Candles”, @spend=10.0,  
> > @prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
> > #\<Treatlistitem:0x234cf90 @longname=“Eye Brow Shape”, @spend=10.0,  
> > @prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
> > #\<Treatlistitem:0x2349db8 @longname=“Full Leg Wax DSS”, @spend=18.0,  
> > @prodtreat=“T”, @numsold=2, @unitcost=10.0\>,  
> > #\<Treatlistitem:0x23476bc @longname=“Bridal Make Up”, @spend=10.0,  
> > @prodtreat=“T”, @numsold=1, @unitcost=10.0\>,  
> > #\<Treatlistitem:0x2342fcc @longname=“Collin Face Powder”, @spend=10.0,  
> > @prodtreat=“P”, @numsold=1, @unitcost=10.0\>]\>

> > @breakdown[2].items.sum{|item| item.spend if item.prodtreat == “P”}

NoMethodError: You have a nil object when you didn’t expect it!  
You might have expected an instance of Array.  
The error occurred while evaluating nil.+  
from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/  
active\_support/core\_ext/enumerable.rb:63:in `sum' from (irb):68:in`inject’  
from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/  
active\_support/core\_ext/enumerable.rb:63:in `each' from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/ active_support/core_ext/enumerable.rb:63:in`inject’  
from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/  
active\_support/core\_ext/enumerable.rb:63:in `sum' from /Library/Ruby/Gems/1.8/gems/activesupport-2.2.2/lib/ active_support/core_ext/enumerable.rb:61:in`sum’  
from (irb):68

This is confusing me mightily!

---

<div class="post-metadata">

**Author:** ![Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/dbc845/32.png) [@Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/u/Ruby_on_RailsS_Talk)\
**Post date:** [August 27, 2009, 9:05am UTC](https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461/3 "2009-08-27T09:05:59Z")

</div>

2009/8/26 Ruby on Rails: Talk [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> @prodtreat=“T”, @numsold=1, @unitcost=10.0\>,
> 
> > > @breakdown[1].items.sum{|item| item.spend if item.prodtreat == “P”}  
> > > active\_support/core\_ext/enumerable.rb:63:in `each’  
> > > value?! Any ideas please?  
> > > A bit of a guess here but it may be that fact that this will attempt  
> > > to include some nil values in the sum as the result of the block will  
> > > be nil (I think) for values that do not match. Perhaps this is ok if  
> > > the first record is included in the sum but later ones not (so it is  
> > > performing value + nil) but fails if the first one is not included in  
> > > the sum (so it performs nil + value). Just a guess.

Colin

---

<div class="post-metadata">

**Author:** ![Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/dbc845/32.png) [@Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/u/Ruby_on_RailsS_Talk)\
**Post date:** [August 27, 2009, 9:41am UTC](https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461/4 "2009-08-27T09:41:06Z")

</div>

On Aug 27, 8:03 am, Colin L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> the sum (so it performs nil + value). Just a guess.

That is indeed it - the value of an if statement where the condition  
is not met. One way of solving this would be to change the code to

if …  
item.spend  
else  
0  
end

Fred

---

<div class="post-metadata">

**Author:** ![Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/dbc845/32.png) [@Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/u/Ruby_on_RailsS_Talk)\
**Post date:** [August 26, 2009, 11:51pm UTC](https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461/5 "2009-08-26T23:51:08Z")

</div>

Looks like the following works:

@breakdown[2].items.find\_all {|item| item.prodtreat == “T”}.sum{|item|  
item.spend}

Really have no idea why? So any help would be enlightening.

Thanks

Darren

PS. Have a feeling that this can be simplified to:

@breakdown[2].items.find\_all {|item| item.prodtreat == “T”}.sum  
(&:spend)

---

<div class="post-metadata">

**Author:** ![Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/dbc845/32.png) [@Ruby\_on\_RailsS\_Talk](https://www.ruby-forum.com/u/Ruby_on_RailsS_Talk)\
**Post date:** [August 28, 2009, 4:44pm UTC](https://www.ruby-forum.com/t/sum-wont-seem-to-add-the-fields/174461/6 "2009-08-28T16:44:25Z")

</div>

Hi,

I have pasted a simplified version of the problem you are trying to  
understand. You cannot do a ‘sum’ on a collection that has a ‘nil’ as  
one of its elements. If ‘nil’ happens to be the first element, you  
will get the ‘NoMethodError’ , as the method ‘+’ is called on it (as  
shown in the error trace ‘nil.+’) . Also note that the elements of the  
collection have to define the method ‘+’ for sum to work.

‘sum’ works by using ‘inject’ which in turn takes the first element  
(unless an initial value is provided) and calls + on it using the next  
element in collection as the input, then takes the result and repeats  
the process for all the rest of the elements

> > ar = [1,2,3]  
> > =\> [1, 2, 3]  
> > ar.sum  
> > =\> 6  
> > ar = [nil,2,3]  
> > =\> [nil, 2, 3]  
> > ar.sum  
> > NoMethodError: You have a nil object when you didn’t expect it!  
> > You might have expected an instance of Array.  
> > The error occurred while evaluating nil.+  
> > from /Library/Ruby/Gems/1.8/gems/activesupport-2.3.3/lib/  
> > active\_support/core\_ext/enumerable.rb:63:in `sum' from (irb):16:in `inject’  
> > …

regards,  
mvp

On Aug 26, 10:00 pm, “Ruby on Rails: Talk” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)